1. Field of the Invention
The present invention relates to a lithium ion secondary battery, and more particularly, to a lithium ion secondary battery in which the impregnation property of an electrolyte is improved.
2. Description of Related Art
Recently, compact and light electrical and electronic apparatuses such as mobile telephones, notebook computers, and camcorders have been actively developed and produced. Battery packs are built in the portable electrical and electronic apparatuses so that the portable electrical and electronic apparatuses can used in a place where an additional power source is not provided. The built-in battery pack includes at least one battery that outputs a voltage of a uniform level in order to drive the portable electrical and electronic apparatus for a predetermined period.
A secondary battery that can be charged and discharged is recently used as the battery pack in consideration of economics. The secondary batteries include a nickel-cadmium (Ni—Cd) battery, a nickel-hydrogen (Ni—H) battery, a lithium metal battery, and a lithium ion battery.
The Ni—Cd battery and the Ni—H battery have been widely used as power sources of portable electronic apparatuses. The lithium ion secondary battery, however, has many advantages over the Ni—Cd battery and the Ni—H battery. The operation voltage of the lithium ion secondary battery is commonly 3.6V, which is three times higher than the operation voltages of the Ni—Cd battery and the Ni—H battery. The energy density per unit weigh of the lithium ion secondary battery is high. Therefore, the use of the lithium ion secondary battery rapidly increases.
In the lithium ion secondary battery, lithium based oxides are used as positive electrode active materials and carbon materials are used as negative electrode active materials. In general, the lithium ion secondary battery is divided into a liquid electrolyte battery and a polymer electrolyte battery depending on the kind of an electrolyte. The battery in which the liquid electrolyte is used is referred to as a lithium ion battery, and the battery in which the polymer electrolyte is used is referred to as a lithium polymer battery. Also, the lithium ion secondary battery is manufactured to have various shapes such as a cylinder type, a polygon type, and a pouch type.
In general, the cylinder type lithium ion secondary battery includes an electrode assembly fabricated by cylindrically winding a positive electrode plate coated with positive active materials, a negative electrode plate coated with negative active materials, and a separator positioned between the positive electrode plate and the negative electrode plate. The separator prevent a direct contact between the positive electrode plate and the negative electrode plate, which otherwise would cause a short, but lithium ions could move through the separator. The cylinder type lithium ion secondary battery also includes a cylinder type case in which the electrode assembly is accommodated, an electrolyte injected into the cylinder type case in which the lithium ions move, and a cap assembly coupled with the upper end of the cylinder type case.
The cylinder type lithium ion secondary battery is manufactured by the following method. First, a positive electrode plate, a negative electrode plate, and a separator between the positive and negative electrodes are laminated together. The positive electrode is coated with a positive active materials, and connected to a positive electrode tab. The negative electrode plate is coated with the negative active materials and connected to a negative electrode tab. The laminated plate, which includes layers of the positive and negative electrode and the separator, is cylindrically wound to manufacture an electrode assembly. Then, the cylinder type electrode assembly is inserted into a cylinder type case so that the electrode assembly is fixed in the cylinder type case. Electrolyte is injected into the cylinder type case, and then, the cylinder type case is sealed by a cap assembly to complete the cylinder type lithium ion secondary battery.
Before inserting the electrode assembly into the cylinder type case, a lower insulating plate is inserted in order to insulate the electrode assembly from the cylinder type case. After inserting the electrode assembly into the cylinder type case and before sealing the cylinder type case, an upper insulating plate is inserted in order to insulate the electrode assembly from the cap assembly.
In the case of the polygon type lithium ion secondary battery, an insulating case for supporting the cap assembly and a lower insulating plate are inserted. The insulating case insulate the electrode assembly from a terminal plate, and the lower insulating plate insulate the electrode assembly from a polygon type case.
Since the insulating plates are commonly formed of polyethylene (PE) or polypropylene (PP), the insulating plates do not have an affinity for the electrolyte. Therefore, the electrode assembly is not sufficiently impregnated with the electrolyte because of the insulating plates that have no affinity for the electrolyte. In the conventional upper insulating plate, holes are formed on the insulating plate around an opening into which a center pin is inserted. However, the size of holes are not sufficient for the electrolyte to freely flow to the electrode assembly positioned under the upper insulating plate. The holes around the opening may be clogged with the electrolyte preventing the flow of electrolyte. Also, the lower insulating plate is attached to the electrode assembly, so that the electrode assembly is not sufficiently impregnated with the electrolyte contained in the lower part of the case.
Furthermore, the density of the electrode assembly increases proportional to the capacity of the battery, so that the external diameter of the electrode assembly increases. When the external diameter of the electrode assembly increases, a space between the cylinder type case and the electrode assembly is reduce, so that it is difficult to impregnate the electrode assembly with the electrolyte. Therefore, it is necessary to form the upper insulating plate and the lower insulating plate in a manner that the electrode assembly is easily impregnated with the electrolyte.
A sealing tape is attached to the outer circumference of the electrode assembly in order to support and protect the wound electrode assembly. Since the sealing tape is commonly formed of polyethylene (PE) or polypropylene (PP), the sealing tape does not have an affinity for the electrolyte. In the case of the cylinder type lithium ion battery (or the polygon type lithium ion battery), the case is made of metals such as aluminum, steel, and alloys. The inner surface of the case has an affinity for the electrolyte unlike the sealing tape.
When the electrode assembly is inserted into the cylinder type case, the outer circumference of the electrode assembly where the sealing tape is wound contacts the inner surface of the case. Therefore, the electrode assembly is prevented from being impregnated with the electrolyte due to the sealing tape that does not have an affinity for the electrolyte. Furthermore, as the capacity of the battery increases, the density of the electrode assembly increases, so that the degree of contact between the case and the sealing tape increases. In such a case, it is necessary to change the shape of the inner surface of the case to make the electrode assembly quickly impregnated with the electrolyte.